Field of the Invention
The present invention relates generally to traffic collision warning devices for detecting and locating moving objects suitably equipped with transponders. More particularly, it relates to a low-cost passive airborne collision warning system (PACWS) and method for tracking nearby aircraft for use in collision avoidance.
It has long been recognized that the potential for aircraft collisions increases substantially in area of high traffic density. The tremendous growth in air travel in the 1960s led to an awareness that something should be done in order to prevent mid-air collisions that were often catastrophic. In response the civil aviation authorities mandated the use of a collision avoidance system in the early 1970s for all aircraft flying in controlled airspace generally known as collision avoidance systems such as the National Air Traffic Control Radar Beacon System. The system enables control towers to determine the heading and location of all transponder-equipped aircraft flying in its controlled airspace. The transponders, which are required to be carried by all aircraft flying in controlled airspace, respond to interrogation signals transmitted from ground-based rotating secondary surveillance radars (SSRs). The interrogated transponder responds by broadcasting a coded signal containing information related to the aircraft, such as its 4-digit ID operating in Mode A or its ID and altitude information operating in Mode C. In countries such as Germany for example, use of Mode S capable transponders is required that enable a ground-air-ground data link to be established to provide support for automated air traffic control in heavy air traffic environments.
Interrogation signals from the rotating SSR are highly directional and are comprised of a series of three pulses separated by a specific delay that are transmitted on a carrier frequency of 1030 MHz, whereas the transponder signals are omni-directional and transmit on 1090 MHz. The SSRs are equipped with a phased array antenna in which the interrogation signals are transmitted on a narrow rotating main beam (typically about 1 complete revolution per 5–12 seconds) that is accompanied by a number of side lobes that have relatively lower signal power. The delay between the pulses specifies the information the transponder should transmit. The amplitude of the pulses are compared to ensure that transponder responds to interrogation by the main beam and not from the side lobes.
FIG. 1 shows a graphic depiction of the interrogation and reply signals according to TSO-C47c specification of the internationally standardized Air Traffic Control Radar Beacon System (ATCRBS). There are several interrogation modes, the most common being Mode A that is a request for an identification code, and Mode C that also asks for the altitude of the responding aircraft. Mode B is currently not used in U.S. operations and Mode D is unassigned at the present time. As can be seen from the figure, the distance between two pulses determines the Mode of interrogation and the range to the aircraft is determined by the time delay. These systems typically have ranges up to at least 100 nautical miles. The transponder reply signals received by the control tower and plotted on a tracking screen and updated frequently to enable the air traffic controller to constantly track all aircraft in its assigned air space. It is then up to the controller to interpret and assess the risk of a collision which he/she attempts to prevent by communicating with the pilots by radio.
There have been many attempts in the past to further improve on these collision avoidance systems. One such system is the Traffic/Airborne alert and Collision Avoidance System (TCAS/ACAS) as proposed by the U.S. Federal Aviation Administration. TCAS II is currently required in the United States on all commercial aircraft having more than 30 seats. Many other countries already have or will likely mandate the use of airborne collision avoidance systems in the near future. TCAS essentially involves an airborne SSR-like system that is capable of actively interrogating surrounding transponder-equipped aircraft with in order to elicit information coded replies that can alert the pilot to the presence of nearby aircraft.
FIG. 2 is a schematic view of an exemplary airborne TCAS/ACAS system. The airborne TCAS/ACAS on the observer aircraft sends out a coded interrogation signal Q1 that is received by transponder-equipped aircraft A1 and A2. The transponders are responsive to the interrogations and transmit replies R1 and R2 respectively on 1090 MHz. The observer aircraft receives the replies and determines whether the aircraft poses a threat of a collision. However, fully equipped systems such as these are quite expensive are more suitable for use with large commercial aircraft since they can run into the hundreds of thousands of dollars.
There are products on the market that provide “lower” cost traffic avoidance systems for use with smaller aircraft. Some of these systems operate on the principle of passively detecting nearby threatening aircraft by analyzing their transponder replies in response to interrogations by the SSR. However, the costs of many of these systems are typically in the range of tens of thousands of dollars, which is still a bit too costly to encourage widespread use by light aircraft that are exempt from the regulations.
U.S. Pat. No. 4,027,307 issued to Lichford describes a collision avoidance and proximity warning system for passively determining the range and bearing of nearby aircraft within a selectable proximity to the observer's aircraft. In the method, the observer's aircraft listens for replies of nearby aircraft to the same interrogation to which its own transponder has just replied and determines the bearing of the intruder aircraft with respect to the axis of the observer's aircraft. However, as described on column 5, lines 11–19, an aircraft that intrude upon the listen-in region will be detected but an aircraft outside this region will not be detected. Thus the limited scope of detection of the method could lead to a failure to detect potentially threatening aircraft flying toward the observer's aircraft.
U.S. Pat. Nos. 5,077,673 and 5,157,615 issued to Brodegard et al. and assigned to Ryan International Corp. are related patents issued to the same assignee that describes a collision avoidance device mounted in an aircraft and operates by listening to replies from other transponder carrying aircraft responding to SSR interrogations. The method, as stated in column 7, lines 15–41 of the '673 patent and similarly stated in the '615 patent, does not attempt to “establish precise range parameters” between a potential threat aircraft to the host aircraft. Instead, the primary parameter used is altitude detection with the idea that a collision between aircraft is not possible unless they are at or near the same altitude. Furthermore, changes in amplitude of the received signal are analyzed with the idea that increasing amplitude indicates that the traffic is closing in distance and thus a potential threat may exist. This method detects when an aircraft enters a potentially threatening zone around the host aircraft but does not produce sufficient information to accurately display the threatening aircraft's position and bearing to better assist the pilot in determining the best maneuver to avoid a collision.
In view of the foregoing, it is desirable to provide a low-cost airborne collision warning device and method that suitable for use in light aircraft that enables accurate determination of information such as range, and bearing, speed etc. to track nearby aircraft for collision avoidance.